1. Field of Invention
This invention relates to a new and improved control system for a resonant free-piston, variable stroke compressor and for a heat pump utilizing such compressor.
More particularly, the invention relates to a control system for controlling automatically the operation of a mechanical resonant free-piston compressor driven by an electrodynamic linear reciprocating motor. The spring-mass system of the resonant free piston compressor (RPC) which is forced into vibration by the linear motor, is designed to have a selectively variable mechanical resonant frequency generally centered on the power line frequency of the alternating current (A.C.) power source used to drive the linear reciprocating motor. The RPC may also include means responsive to any over-stroking of the piston which are operative to vary the natural frequency of the spring-mass system by varying the stiffness of resilient gas springs provided in the RPC.
2. Background of Invention
A controlled stroke, resonant free-piston compressor employing an electrodynamic linear motor is described and claimed in U.S. Pat. No. 3,937,600 issued Feb. 10, 1976, Harlan V. White--Inventor--entitled "Controlled Stroke Electrodynamic Linear Compressor". An improvement in that compressor is described in copending U.S. application Ser. No. 160,429 (HD-5454). Peter W. Curwen and Richard A. Dorman, Inventors filed concurrently with this application, entitled "Resonant Piston Compressor Having Improved Stroke Control", and in U.S. Pat. No. 4,067,667 for a "Controlled Stroke Electrodynamic Oscillating Motor Compressor", Harlan V. White--Inventor all assigned to Mechanical Technology, Incorporated of Latham, New York. The disclosures of each of these prior art reference United States patents and copending application, are hereby incorporated in their entirety into this application.
RPC heat pumps, for the purposes of this disclosure, are defined as load-following (or modulating) heat pumps where the words "load-following" or "modulating" mean that the output heating or cooling capacity of the heat pump is varied in a continuous manner so that its output capacity matches the actual heating/cooling load requirement. For example, if the load is a single-family residence where the heating or cooling requirements vary as a function of outdoor weather conditions, the heating or cooling capacity of an RPC heat pump automatically will be smoothly varied (modulated) to maintain the temperature of the house at a desired temperature set by a thermostat control within the house. As a consequence of its load-following operating characteristics, the RPC heat pump will operate more or less continuously. This is in sharp contrast to the on-off cycling type of operation characteristic of most currently available (non-modulating) heat pumps. It has been determined, as a result of the above briefly described experience, that the modulating mode type of heat pump operation is more energy efficient than on-off cycling and can result in significant kilowatt-hour savings (operating cost savings) for the heat pump owner.
The RPC heat pump modulating capability is attained by means of a unique linear reciprocating motor-driven resonant free-piston compressor. To obtain variable capacity operation, the displacement of the RPC is changed by changing or varying piston stroke, hereinafter referred to as stroke modulation.
Studies indicate that by providing a control capability for finely adjusting piston stroke (fine stroke modulation), RPC-equipped heat pumps should yield significant energy savings and fast pay back of installed cost premiums. Compared to the best residential heat pumps currently available (those heat pumps with constant-displacement compression operating in an on-off cycling mode), the advantages of a stroke modulation RPC heat pump predictably are as follows: a 19-34% reduction in annual operating costs for the owner, depending upon geographic location; a 2.2 years or less pay back time on installed costs premium, again depending upon geographic location; higher inherent reliability resulting from (1) RPC tolerance to liquid refrigerant flood back, and (2) greatly reduced number of heat pump on-off and defrost cycles as a consequence of modulation, and lower maintenance and repair costs due to reduced stress as a consequence of the stroke modulation control technique in contrast to the higher stress on-off cycling of conventional heat pump components.
In order to take full advantage of the stroke-modulation capability inherent in the RPC devices described in the above-listed patents, it was necessary to provide some means for better controlling (modulating) the resonant free-piston stroke whereby finer control over displacement of the compressor and its operation in a heat pump system, could be maintained. For this purpose, the present invention was devised.